1. Field of the Invention
The present invention relates generally to the magnetic information storage technology, and particularly, to magnetic recording disc drives including a sensor having a giant magnetoresistance (GMR) based spin valve structure or a tunneling magnetoresistance(TMR) based magnetic tunnel junction structure or magnetic random access memory device including a magnetic memory element(corresponding to a capacitor of DRAM) having a giant magnetoresistance (GMR) based spin valve structure or a tunneling magnetoresistance(TMR) based magnetic tunnel junction structure.
More particularly, the present invention relates to a spin valve magnetoresistive structure employed in the sensor of magnetic recording disc drive or tunnel junction magnetoresistive structure employed in the magnetic memory element of magnetic random access memory device.
2. Description of the Related Art
Spin-valve (SV) multilayers possessing giant magnetoresistance (GMR) are widely used in magnetic recording disc drives and are also considered for magnetic random access memory (MRAM) devices for high density data storage.
The magnetostatic field (Hms) generated from pinned layer of the SV multilayers should be reduced in order to improve the bias point (BP) and the sensitivity of SV. This becomes more prominent when the unit cell size becomes smaller because the contribution of the magnetostatic interactions to the total energy increases with a decrease of the cell size. To solve this problem, Fujitsu and Seagate used the bias compensation layer adjacent to free and pinned layer, respectively, and IBM used the synthetic antiferromagnet-based spin-valves (SSV) to reduce the net moment of the pinned layer.
A magnetic bias point control is of practical importance for operating magnetic recording disc drives and magnetic random access memory devices. The magnetic bias point is defined as an offset of the central position of the magnetoresistive transfer curve with respect to the zero field. The magnetoresistive transfer curve is usually linear for magnetic recording disc drives and square-shaped for magnetic random access memory applications.
FIG. 1 shows a conventional synthetic antiferromagnet-based spin-valves(SSV) structure. In this regard, U.S. Pat. No. 5,701,223 discloses a inverted spin valve magnetoresistive sensor which is used in magnetic recording disc drive.
A free layer is separated from a synthetic antiferromagnetic structure(pinned layer) by a nonmagnetic, electrically-conducting spacer layer (e.g. Cu) in FIG. 1. The magnetization of the synthetic antiferromagnetic structure is fixed by an antiferromagnetic layer(AFM layer). The synthetic antiferromagnetic structure(pinned layer) comprises a first ferromagnetic layer (P1) and a second ferromagnetic layer (P2) separated by an antiparallel coupling layer (APC) of nonmagnetic material. Typical material used as P1 (or P2) is CoFe and typical material used as APC is Ru. The two ferromagnetic layers in the synthetic antiferromagnetic structure(pinned layer) have their magnetization directions oriented antiparallel. The free layer is formed on a seed layer deposited on the substrate. To complete the spin valve(SV) magnetoresistive structure, a capping layer is formed on the AFM layer. Typical material used as a seed layer is NiFe, Cu or Ru. In particular, SSV with CoFe (P2)/Ru/CoFe (P1) trilayer pinning schemes possesses the advantages of improved thermal and magnetoresistive properties.
They, however, have still suffered from poor switching asymmetry at submicrometer cell size due to the thickness difference between P2 and P1 layers. Also, as cell size decreased, the bias point in the SSV increased exponentially due to a magnetostatic field from the CoFe layer.
Moreover, the effective exchange field is needed much larger than that of the SSV. The shape of magnetoresistive transfer cures is needed to be maintained in spite of the device size variation.